The term laminated pane is to be understood, according to this invention, as implying a pane composed of at least one rigid sheet and of one flexible sheet of plastics material. These laminated panes may be panes known as symmetrical, when they comprise two rigid sheets as outer sheets. These laminated panes may also be known as asymmetrical, when they comprise a rigid sheet and a flexible sheet as outer sheets.
These sheets of flexible plastics material forming part of the composition of a laminate d pane are intended to improve the properties of a pane. For example, it may be a plastics material of the polyurethane type, used as outer sheet in order to give the pane, notably, properties of resistance to scratching. They may also be intermediate sheets of polyurethane or polyvinyl butyral or any other material. An intermediate sheet of polyvinyl butyral improves, in particular, the impact resistance of the pane.
Rigid sheets may also undergo treatments for the purpose of improving their characteristics. It is known, for example, to subject glass sheets to a chemical or thermal toughening for increasing their mechanical strength.
These laminated panes may comprise a decorative and/or functional motif.
This may be a motif for decorating the pane and/or enabling it to harmonize with its environment. The motif may also serve for protecting, notably from light, the adhesive films used for mounting the pane or for the fitting of accessories, or again for masking the electrical supply strips for heating networks.
Whatever its function, this motif is visible.
For this reason, the motif should be free of any deterioration, throughout its entire existence, which may extend to ten years or more. The term deterioration is to be understood, according to this invention, as including any scratching, any flaking, scaling or chipping, any overflowing or again any alteration in the colors of the motif, its shape etc. The flow or creep of the material constituting the motif leads, for example, to overflowing or inaccuracies of shape.
For all these reasons, such a motif is generally composed of an enamel, the excellent heat resistance and mechanical strength of which is well known. Moreover, enamel has the advantage of possessing a high adhesive power with respect to glass: its composition includes fritts of glass capable of being vitrified at high temperature and thus intimately bonding the enamel to the glass support.
To produce the enamelled layer, the preferred procedure is to deposit the enamel on the substrate, and then to dry the humid film formed until this film has sufficient adhesion and strength to avoid the appearance of marks on the formed film during the handling of the pane.
Different films can, of course, be deposited in this way. Finally, the enamelled film or films are subjected to a thermal treatment at high temperature in order to vitrify the enamel and to obtain the definitive coating.
For reasons of expenditure of energy, this thermal treatment at high temperature is generally associated with the thermal treatments used for transforming the glass sheet, for example for the bending or the thermal toughening of the glass sheet.
This method gives entire satisfaction when it is intended to be applied to a bent and/or thermally toughened pane, which may perhaps be laminated.
However, it requires a supplementary and expensive thermal treatment when the pane is not of curved shape nor is thermally toughened.
Furthermore, the enamelling process is incompatible with a chemical toughening treatment. The enamelled coating forms a kind of barrier resisting the ion exchanges for the purpose of chemically reinforcing the surface of the glass supporting the enamel film. On the other hand, the enamelling process if carried out after the chemical toughening treatment harms the toughening of the glass.